This invention relates to cycloconverters with intermediate d-c links in general and more particularly to an improved commutation circuit for such a cycloconverter.
Such cycloconverters with variable intermediate link voltage are known from the book by M. Mayer "Self-Commutating Thyristor Converters," Siemens A/G 1974, third edition, pages 162 to 168, and from the German Pat. No. 1,246,861. (See also Thyristor Phase-Controlled Converters and Cycloconverters by B. R. Pelly, published by Wiley Interscience-a division of John Wiley & Sons, Inc.). Because of their simplicity, they find widespread application, e.g., for supplying three phase machines. In these cycloconverters, a commutating capacitor is recharged after each forced commutation with the energy stored in the commutating inductance by the load current and from the energy of the intermediate link to cover the commutation losses. As a result, the ability to commutate remains assured even if the intermediate link voltage is small. With large load currents, particularly if the cycloconverter is overloaded, this current dependent recharging can lead to a large increase of the voltage at the commutating capacitor, i.e., the commutation voltage, and thereby to excessive stresses of the thyristors, the diodes and the capacitors in the cycloconverter. This danger exists particularly in the case of high power, e.g., powers of 100 kVA and more.
Thus, there is a need for a cycloconverter of the general type mentioned above which includes a commutation circuit designed in such a manner that excessively high commutation voltages are avoided.